The invention relates to a method for monitoring the operating state of a load which is driven cyclically by a control signal, especially for an electric motor in a motor vehicle, the cyclic drive being provided by a pulse-width-modulated drive signal, and to a circuit arrangement for implementing the method.
During the dynamic operation of loads, such as an electric motor in a control loop, the load is driven with pulse-width-modulated signals. During this dynamic operation it is necessary to accelerate the electric motor, to brake it or else to change the direction of rotation.
In the event of an abrupt change of direction on the electric motor, a reversing current is produced which, for a short time, may be several times the operating current of the electric motor. In order to prevent overloading of the power source and the electronics for this case, it is usual to provide electronic current limiters.
In order to protect the electronics from a short circuit, electric short-circuit protection is provided, whose threshold must lie above the high reversing current.
However, because of the real internal resistances of the electronics, at low operating voltages and in the case of a non-ideal short circuit (R.sub.1 &gt;30 m.OMEGA.) or a stalled motor, a current flow which leads to the short-circuit protection responding is not reached. In this case because of the finite current rise, the current limiter responds only with a delay. Since the current limiter is reset again during each PWM period, a high, pulsed current flows through the output stage over the whole time, which leads to the electronics being destroyed or produces the risk of a cable fire.